Work experience is education. That's why you get paid more for past experience - it saves the company on a cost they would otherwise have invested in you. Doing a PhD is just another way of getting work experience, except you get paid a lot less.

I think it's reasonable to claim that PhD's are compensated a lot by virtue of the "cost" of educating them and other benefits. But this education is something you receive in a normal job too. And just like in a normal job, the employer gains a lot more out of the exchange. Grad students are typically worth a lot more to universities than they cost, otherwise there would be no sense in admitting so many. I'd wager the ratio of value to cost is perhaps greater in academia than it is in a lot of other industries.